Ah, you said it! It was FidoNet. As for hosts running BBS server software in early-90's, I think OS/2 was quite popular choice as it was generally seen so stable and solid system. (plus it was multitasking nicely which wasn't obvious then!)

I don't know how many IRC (or other more recent chatting technologies based on IRC) users know this, but IRC has BBS background as well:
ircnet.org/…